The Supreme Court on Wednesday indefinitely blocked cameras from covering the high-profile federal court trial on the constitutionality of California’s ban on same-sex marriage.

Now in its third day, the trial in federal court in San Francisco concerns Proposition 8, the voter-approved ban on gay marriage in the state.

The presiding trial judge, Vaughn R. Walker, had authorized real-time streaming of the proceedings for viewing in other federal courthouses in California, New York, Oregon and Washington and contemplated posting recordings of the trial on the court’s Web site after several hours of delay.

However, a conservative 5-4 majority ruled against streaming the trial to the other courts. The high court said it wasn’t deciding the Web site posting plan because Walker hadn’t formally requested authorization for that proposal.

Gay rights advocates condemned the ruling sought by lawyers representing proponents of gay marriage who were concerned that broadcasts would expose their trial witnesses to retaliation from gay marriage supporters.

“The Supreme Court just struck a huge blow against transparency and accountability,” said Rick Jacobs, chairman of the Courage Campaign, a Los Angeles-based gay rights organization. “This historic trial will remain largely hidden from public view, despite it’s historic potential to challenge and change the minds of Americans.”

In federal court Wednesday, evidence in the case targeted a proponent of Proposition 8 who warned voters in a letter during the 2008 campaign that gay rights activists would try to legalize sex with children if same-sex couples had the right to wed.

San Francisco resident Hak-Shing William Tam, a defendant in the lawsuit, discussed the letter sent to Chinese-Americans church groups during a legal deposition taped last month.

“On their agenda list is: legalize having sex with children,” states the letter, which also cautioned that “other states would fall into Satan’s hands” if gays weren’t stopped from marrying in California.

Lawyers for the two same-sex couples pursuing the lawsuit introduced the footage in court to buttress their contention that Proposition 8 is unconstitutional because it was fueled by deep-seated animosity against gays.

Regardless of the outcome of the lawsuit, the case is likely to be appealed to the U.S. Supreme Court, where it could lead to laws that restrict marriage to a man and a woman being upheld or abolished nationwide.

In its unsigned opinion about cameras, the Supreme Court criticized Walker for attempting to change the rules “at the eleventh hour to treat this case differently than other trials.

“Not only did it ignore the federal statute that establishes the procedures by which its rules may be amended, its express purpose was to broadcast a high-profile trial that would include witness testimony about a contentious issue.”

The court’s majority, though not identified in the ruling, consists of Chief Justice John Roberts and Justices Samuel Alito, Anthony Kennedy, Antonin Scalia and Clarence Thomas.

The four justices in dissent with the ruling were Stephen Breyer, Ruth Bader Ginsburg, Sonia Sotomayor and John Paul Stevens. In a dissent written by Breyer, they said the high court should have stayed out of the issue.

Breyer said “the public interest weighs in favor of providing access to the courts.”

In his San Francisco courtroom, Judge Walker said the ruling provided “limited guidance,” and the Web site issue was pending.

“My inclination,” he said, “is to put that issue aside for the time being.”

Walker said he didn’t want to delay the progress of the trial.

Anthony Pugno, a Sacramento lawyer for the group behind the ballot initiative, said he was puzzled by Walker’s comments about the possibility of posting video on the Web.

Most federal courts say they fear broadcasts will diminish the system’s dignity, could unfairly influence rulings and disrupt proceedings. There is also concern that judges, lawyers and witnesses will pander to the camera while potential jurors will shy away from serving out of concern they will be identified.

Though all 50 states allow cameras into some state-level court proceedings, federal courts from the high court on down have for decades generally refused to admit cameras into courtrooms.

Congress has failed several times to pass bills introduced to specifically allow the technology in the federal courts, though a new proposal with bipartisan support to allow cameras is pending before the U.S. Senate’s Judiciary Committee.

Six trial courts and two appeals court participated in a three-year study during the 1990s that included granting applications to broadcast 186 hearing, including 56 trials.

A majority of judges and lawyers who participated in the program supported opening federal courts to cameras, but the Judicial Conference of the United States – which sets the court system’s policies – still said it was against recording hearings and trials.

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Sherman reported from Washington. Associated Press Writer Lisa Leff in San Francisco contributed to this report.